Conventionally, due to a high absorption rate with respect to aqueous liquid, a great absorption amount, and a great retention property, a water absorbent resin is blended with a fibrous material as necessary so as to constitute an absorbent core of a sanitary material for a purpose of use in a sanitary material such as a diaper.
Recently, with requirement for reduction in thickness of a sanitary material such as a diaper, a ratio of the water absorbent resin in the absorbent core is likely to increase (concentration is likely to be higher) (for example, see Patent Documents 1 to 3). As performances of the water absorbent resin used in the higher-concentration absorbent core in which the ratio of the water absorbent resin is high, an inter-particle void average radius at the time of saturation and swelling without pressure and an inter-particle void average radius at the time of saturation and swelling under pressure are defined (see Patent Documents 4 and 5). Besides, a large number of water absorbent resins suitable for use with high concentration have been proposed (see Patent Documents 6 to 15). However, there is a case where even the water absorbent resin satisfying the proposed properties does not sufficiently meet the requirement in terms of its performance at the occasion of use in a more highly concentrated sanitary material such as a diaper.
[Patent Document 1]
International Publication No. 95/26209 Pamphlet
[Patent Document 2]
U.S. Pat. No. 5,669,894
[Patent Document 3]
U.S. Pat. No. 5,599,335
[Patent Document 4]
Japanese Unexamined Patent Publication No. 290290/2003 (Tokukai 2003-290290)
[Patent Document 5]
U.S. Unexamined Patent Publication No. 2003-0181115
[Patent Document 6]
U.S. Pat. No. 5,149,335
[Patent Document 7]
U.S. Pat. No. 5,601,542
[Patent Document 8]
U.S. Pat. No. 6,414,214
[Patent Document 9]
U.S. Pat. No. 6,849,665
[Patent Document 10]
U.S. Pat. No. 7,098,284
[Patent Document 11]
U.S. Reissue Pat. No. 38444
[Patent Document 12]
U.S. Pat. No. 5,797,893
[Patent Document 13]
U.S. Pat. No. 6,300,275
[Patent Document 14]
U.S. Pat. No. 6,831,142
[Patent Document 15]
U.S. Pat. No. 6,930,221